The down-regulation of gap junctions has often been associated with cellular transformation and tumorigenesis. In cell culture, closure of connexin 43 (Cx43) channels was found to be an early event in response to activation of oncogenes or growth factor receptors, preceding changes in morphology and growth control. The aim of this study is to systematically characterize the different events in transformation by one particular oncogene, v-src, that has been shown to induce Cx43 uncoupling and to determine to what extent Cx43 uncoupling influences transformation. v-Src uncouples Cx43 gap junctions through activation of the Ras/Raf/MEK/MAPK pathway. MAPK has been demonstrated to directly phosphorylate Cx43 at serine sites within its two SH3 binding domains on the carboxy-terminal domain of the protein. Phosphorylation at these serine sites induces closure of the junctional channel through a "ball and chain" mechanism. To eliminate background from endogenous protein, cells that are derived from Cx43 knockout mice will be transfected with temperature sensitive v-src vectors. The various parameters of transformation will be characterized and the importance of MAPK phosphorylation of Cx43 in gap junction uncoupling will be determined. The cells will be transfected with vectors encoding wild type or mutant Cx43 in order to assess the role of gap junction uncoupling in cellular transformation. Based on these results, further experiments are proposed to elucidate the mechanism of Cx43 tumor suppressor activity.